At present many types of pyrotechnic priming charges are found, hereinafter also referred to as primer, for use either in the civil or the military sector. Civil applications can be within the sphere of vehicle safety, for example gas generators for airbags or safety belt stretchers. Military applications can be shells in which the pyrotechnic priming charge constitutes part of an ignition chain, for example in a detonating cap for detonating an initiator, such as lead azide, which in turn sets off an explosive, for example RDX (Hexogen) or HMX (Octogen).
A pyrotechnic priming charge comprises a mixture of at least one reducing agent, hereinafter also referred to as fuel, and at least one oxidizing agent, hereinafter referred to as oxidizer, as well as further additives such as bonding agents and burning rate moderators. The mixture is normally found in powder form, grain form or granulate form and is worked into a solid body in some type of pressing operation, for example ram pressing, extrusion or isostatic pressing. Once the pyrotechnic priming charge has been initiated and the combustion is underway, oxygen for the combustion is taken from the oxidizer.
One problem with pyrotechnic priming charges of the said type is a low oxygen availability. Low oxygen availability means low efficiency in the combustion. The availability of the oxygen depends essentially on the compactness of the oxidizer and the fuel. Small grains of regular form allow a high compactness and thus a short distance between oxygen and fuel. Large grains of irregular form, on the other hand, mean a low compactness between the grains and thus a large distance between oxygen and fuel. In practice, it is difficult, though, to achieve sufficiently small and regular grains. The ideal case is when the oxygen and the fuel form part of the same molecule, for example in molecules such as nitrocellulose, nitroglycerin and hexogen.
It is known to increase the availability of the oxygen in a pyrotechnic priming charge by dissolving the oxidizer in a solvent and subsequently adding the oxidizer solution to a coherent porous fuel structure, see, for example, document US 2003/0148569 A1.
By distilling off the solvent, the oxidizer will be deposited as a fine crystalline layer in the porous fuel structure.
A coherent porous fuel structure can be likened to a sponge in which parameters such as pore size, pore volume and specific surface area can be controlled by varying the production method. An important advantage with porous fuel structures of this type is the large specific surface area. Large specific surface area means that the distance between the oxygen and the fuel can be made short and mixing can occur at virtually molecular level. Porous fuel structures have long been commercially available and can be produced according to various methods, for example by etching. The basic material in the structure, that is to say the fuel, can be constituted, for example, by silicon, but materials such as carbon, aluminium, magnesium and zinc are also used.
In US 2003/0148569 A1, a pyrotechnic priming charge is described, comprising a coherent porous fuel structure filled with oxidizer. The coherent porous fuel structure consists of silicon, and the oxidizer of lithium nitrate or ammonium perchlorate. The primer is produced by a saturated methanol solution of lithium nitrate being fed to the coherent porous silicon structure, after which the solvent is distilled off, whereupon the oxidizer is precipitated as crystals in the porous silicon structure. It is also known from the literature to use other types of oxidizers, for example organic nitro compounds.
Oxidizers of the said types, especially lithium nitrate and ammonium perchlorate, are moisture and temperature sensitive, which means that, after a period of exposure to the ambient atmosphere, they will be broken down. The breakdown of the oxidizer can lead to unintentional ignition of the pyrotechnic priming charge, which can have serious consequences. Furthermore, the use of chlorine-based oxidizers then means that hydrochloric acid is formed in the combustion, which is damaging to human health and the environment.
The technical problem on which the present invention is founded has been the moisture and temperature sensitivity of oxidizers in pyrotechnic priming charges comprising a coherent porous fuel structure, which moisture and temperature sensitivity can lead to breakdown of the oxidizer and unintentional ignition.